A-levels should be made harder, says the education standards watchdog, responding to this summer's improved exam results.

Chris Woodhead, the Chief Inspector of Schools in England, says that the political pressure to encourage more young people to stay in school should not be an excuse for letting standards fall.

Instead of lowering standards to allow more students to take A-levels, Mr Woodhead called for the exam to be targeted at a small number of the most able pupils.

And he questioned the academic rigour of an exam which this year saw almost 90% of entrants passing.

Doug McAvoy says the chief inspector's opinions are "elitist"

But there have been accusations from teachers' leaders that Mr Woodhead's arguments are elitist and that he is undermining the achievements of many pupils.

"Chris Woodhead betrays his fundamentally elitist approach to education. In his view, the benefits of study for university should be confined to the few," said Doug McAvoy, general secretary of the National Union of Teachers.

Another union leader, Nigel de Gruchy of the National Association of Schoolmasters Union of Women Teachers, called Mr Woodhead's comments "negative and sterile".

Writing in the Guardian, Mr Woodhead attacked a system in which every year exam results were expected to rise and which could not accept failure.

"An examination which doesn't involve failure is a very peculiar examination... for me failure isn't a dirty word at all. An education system must involve failure. Life involves failure."

Pupils who were less academically able - and who previously would have left school at 16 - should be encouraged to take vocational courses, he said.

Mr Woodhead also criticised the consensus between education professionals and the government that made a "dissident" of anyone who questioned whether standards were rising in line with the improvements in results.

Exam authorities and the goverment are likely to reject the claim that the "gold standard" of A-levels has been compromised by the increased number of pupils taking the exam.

They have consistently argued that standards have been carefully monitored and that there has been no reduction in the quality of candidates' exam papers and coursework.